Method and apparatus for improving statistical word alignment models

ABSTRACT

A method and computer-readable medium determine word association scores for word pairs. Each word association score is converted into a translation probability using a normalization value that reserves probability mass for unseen data.

BACKGROUND OF THE INVENTION

The present invention relates to statistical word alignment. Inparticular, the present invention relates to training statistical wordalignment models.

In statistical machine translation, parameters are generally trainedthat estimate the probability of a source language word being translatedinto one or more target language words. This translation probability canbe used to estimate the probability of a sequence of words in the targetlanguage given a sequence of words in the source language. For example,under a well known model known as the IBM Model 1, the probability of asequence of words in the target language given the sequence of words inthe source language is estimated as: $\begin{matrix}{{p( {T❘S} )} = {\frac{ɛ}{( {l + 1} )^{m}}{\prod\limits_{j = 1}^{m}{\sum\limits_{i = 0}^{l}{{tr}( {t_{j}❘s_{i}} )}}}}} & {{EQ}.\quad 1}\end{matrix}$where p(T|S) is the probability of a sequence of words in the targetlanguage given a sequence of words in a source language, m is the numberof words in the sequence of target language words, 1 is the number ofwords in the sequence of source language words, E is the probabilitythat a sequence of words in the target language will be m words long,and tr(t_(j)|s_(i)) is the translation probability, which provides theprobability of the jth word in the sequence of target language wordsgiven the ith word in the sequence of source language words.

The translation probabilities can also be used as part of a statisticalword alignment model. Such models are used to identify an alignmentbetween a source sentence and a target sentence, where the alignment isdefined as identifying which source words and target words aretranslations of each other in the two sentences. If the translationmodel is limited such that each target word can be generated by exactlyone source word (including a null word) an alignment a can berepresented by a vector a₁, . . . , a_(m), where each a_(j) is thesentence position of the source word generating target word t_(j)according to the alignment. When this is true, the most likely alignmentâ of a source sentence and a target sentence according to IBM Model 1 isgiven by: $\begin{matrix}{\hat{a} = {\arg\quad{\max_{a}{\prod\limits_{j = 1}^{m}{{tr}( {t_{j}❘s_{a_{j}}} )}}}}} & {{EQ}.\quad 2}\end{matrix}$where s_(a) _(j) is the source word predicted by alignment a_(j) fortarget word tj. The notation argmax_(a)f(a) means the value of a forwhich f(a) has the maximum value.

Before a translation probability can be used in an alignment model or ina translation model, it must first be trained. Under the prior art, suchtranslation models have typically been trained using anExpectation-Maximization (EM) algorithm. This algorithm relies on acorpus of paired sentences, where each sentence pair consists of asentence in the source language and a translation of that sentence inthe target language. During the expectation phase of the EM algorithm,counts are developed for word pairs, where a word pair consists of oneword from the source language and one word from the target language thatoccur together in at least one of the paired sentences. Each occurrenceof the word pair receives a count depending on the probability of thesource word being translated into the target word, according to thecurrent estimate of the translation probabilities.

Initially, each translation probability is set to a uniform distributionover the target language vocabulary. During the maximization phase, thecounts are normalized and a probability is re-estimated for eachtranslation. The process is then repeated using the updated translationprobability estimates. Mathematically, it has been shown that as thenumber of iterations of this process increases, the EM algorithm willconverge on the maximum likelihood estimates for the translationprobabilities.

Under the prior art, this was thought to provide the best set of modelparameters for alignment and translation. However, model parameterstrained in this way have been less than ideal. One reason for this isthat the EM algorithm trains the parameters to best fit the trainingdata. If the training data is not representative of the actual dataencountered during translation or alignment, the algorithm will over fitthe parameters to describe the training data instead of the actual data.

Thus, new techniques are needed to avoid the over-fitting of translationprobability parameters during training.

SUMMARY OF THE INVENTION

A method and computer-readable medium determine word association scoresfor word pairs. Each word association score is converted into atranslation probability using a normalization value that reservesprobability mass for unseen data.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of one computing environment in which thepresent invention may be practiced.

FIG. 2 is a flow diagram of a method of estimating translationprobabilities under one embodiment of the present invention.

FIG. 3 is a flow diagram of a method of estimating translationprobabilities under one particular embodiment of the present invention.

FIG. 4 is a flow diagram of a method of estimating translationprobabilities under an alternative embodiment of the present invention.

FIG. 5 is a flow diagram of a method of estimating translationprobabilities under a further alternative embodiment of the presentinvention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

FIG. 1 illustrates an example of a suitable computing system environment100 on which the invention may be implemented. The computing systemenvironment 100 is only one example of a suitable computing environmentand is not intended to suggest any limitation as to the scope of use orfunctionality of the invention. Neither should the computing environment100 be interpreted as having any dependency or requirement relating toany one or combination of components illustrated in the exemplaryoperating environment 100.

The invention is operational with numerous other general purpose orspecial purpose computing system environments or configurations.Examples of well-known computing systems, environments, and/orconfigurations that may be suitable for use with the invention include,but are not limited to, personal computers, server computers, hand-heldor laptop devices, multiprocessor systems, microprocessor-based systems,set top boxes, programmable consumer electronics, network PCs,minicomputers, mainframe computers, telephony systems, distributedcomputing environments that include any of the above systems or devices,and the like.

The invention may be described in the general context ofcomputer-executable instructions, such as program modules, beingexecuted by a computer. Generally, program modules include routines,programs, objects, components, data structures, etc. that performparticular tasks or implement particular abstract data types. Theinvention is designed to be practiced in distributed computingenvironments where tasks are performed by remote processing devices thatare linked through a communications network. In a distributed computingenvironment, program modules are located in both local and remotecomputer storage media including memory storage devices.

With reference to FIG. 1, an exemplary system for implementing theinvention includes a general-purpose computing device in the form of acomputer 110. Components of computer 110 may include, but are notlimited to, a processing unit 120, a system memory 130, and a system bus121 that couples various system components including the system memoryto the processing unit 120. The system bus 121 may be any of severaltypes of bus structures including a memory bus or memory controller, aperipheral bus, and a local bus using any of a variety of busarchitectures. By way of example, and not limitation, such architecturesinclude Industry Standard Architecture (ISA) bus, Micro ChannelArchitecture (MCA) bus, Enhanced ISA (EISA) bus, Video ElectronicsStandards Association (VESA) local bus, and Peripheral ComponentInterconnect (PCI) bus also known as Mezzanine bus.

Computer 110 typically includes a variety of computer readable media.Computer readable media can be any available media that can be accessedby computer 110 and includes both volatile and nonvolatile media,removable and non-removable media. By way of example, and notlimitation, computer readable media may comprise computer storage mediaand communication media. Computer storage media includes both volatileand nonvolatile, removable and non-removable media implemented in anymethod or technology for storage of information such as computerreadable instructions, data structures, program modules or other data.Computer storage media includes, but is not limited to, RAM, ROM,EEPROM, flash memory or other memory technology, CD-ROM, digitalversatile disks (DVD) or other optical disk storage, magnetic cassettes,magnetic tape, magnetic disk storage or other magnetic storage devices,or any other medium which can be used to store the desired informationand which can be accessed by computer 110. Communication media typicallyembodies computer readable instructions, data structures, programmodules or other data in a modulated data signal such as a carrier waveor other transport mechanism and includes any information deliverymedia. The term “modulated data signal” means a signal that has one ormore of its characteristics set or changed in such a manner as to encodeinformation in the signal. By way of example, and not limitation,communication media includes wired media such as a wired network ordirect-wired connection, and wireless media such as acoustic, RF,infrared and other wireless media. Combinations of any of the aboveshould also be included within the scope of computer readable media.

The system memory 130 includes computer storage media in the form ofvolatile and/or nonvolatile memory such as read only memory (ROM) 131and random access memory (RAM) 132. A basic input/output system 133(BIOS), containing the basic routines that help to transfer informationbetween elements within computer 110, such as during start-up, istypically stored in ROM 131. RAM 132 typically contains data and/orprogram modules that are immediately accessible to and/or presentlybeing operated on by processing unit 120. By way of example, and notlimitation, FIG. 1 illustrates operating system 134, applicationprograms 135, other program modules 136, and program data 137.

The computer 110 may also include other removable/non-removablevolatile/nonvolatile computer storage media. By way of example only,FIG. 1 illustrates a hard disk drive 141 that reads from or writes tonon-removable, nonvolatile magnetic media, a magnetic disk drive 151that reads from or writes to a removable, nonvolatile magnetic disk 152,and an optical disk drive 155 that reads from or writes to a removable,nonvolatile optical disk 156 such as a CD ROM or other optical media.Other removable/non-removable, volatile/nonvolatile computer storagemedia that can be used in the exemplary operating environment include,but are not limited to, magnetic tape cassettes, flash memory cards,digital versatile disks, digital video tape, solid state RAM, solidstate ROM, and the like. The hard disk drive 141 is typically connectedto the system bus 121 through a non-removable memory interface such asinterface 140, and magnetic disk drive 151 and optical disk drive 155are typically connected to the system bus 121 by a removable memoryinterface, such as interface 150.

The drives and their associated computer storage media discussed aboveand illustrated in FIG. 1, provide storage of computer readableinstructions, data structures, program modules and other data for thecomputer 110. In FIG. 1, for example, hard disk drive 141 is illustratedas storing operating system 144, application programs 145, other programmodules 146, and program data 147. Note that these components can eitherbe the same as or different from operating system 134, applicationprograms 135, other program modules 136, and program data 137. Operatingsystem 144, application programs 145, other program modules 146, andprogram data 147 are given different numbers here to illustrate that, ata minimum, they are different copies.

A user may enter commands and information into the computer 110 throughinput devices such as a keyboard 162, a microphone 163, and a pointingdevice 161, such as a mouse, trackball or touch pad. Other input devices(not shown) may include a joystick, game pad, satellite dish, scanner,or the like. These and other input devices are often connected to theprocessing unit 120 through a user input interface 160 that is coupledto the system bus, but may be connected by other interface and busstructures, such as a parallel port, game port or a universal serial bus(USB). A monitor 191 or other type of display device is also connectedto the system bus 121 via an interface, such as a video interface 190.In addition to the monitor, computers may also include other peripheraloutput devices such as speakers 197 and printer 196, which may beconnected through an output peripheral interface 195.

The computer 110 is operated in a networked environment using logicalconnections to one or more remote computers, such as a remote computer180. The remote computer 180 may be a personal computer, a hand-helddevice, a server, a router, a network PC, a peer device or other commonnetwork node, and typically includes many or all of the elementsdescribed above relative to the computer 110. The logical connectionsdepicted in FIG. 1 include a local area network (LAN) 171 and a widearea network (WAN) 173, but may also include other networks. Suchnetworking environments are commonplace in offices, enterprise-widecomputer networks, intranets and the Internet.

When used in a LAN networking environment, the computer 110 is connectedto the LAN 171 through a network interface or adapter 170. When used ina WAN networking environment, the computer 110 typically includes amodem 172 or other means for establishing communications over the WAN173, such as the Internet. The modem 172, which may be internal orexternal, may be connected to the system bus 121 via the user inputinterface 160, or other appropriate mechanism. In a networkedenvironment, program modules depicted relative to the computer 110, orportions thereof, may be stored in the remote memory storage device. Byway of example, and not limitation, FIG. 1 illustrates remoteapplication programs 185 as residing on remote computer 180. It will beappreciated that the network connections shown are exemplary and othermeans of establishing a communications link between the computers may beused.

FIG. 2 provides a flow diagram of a method under the present inventionfor estimating a set of translation probabilities that can be used in atranslation model or an alignment model. This method is built upon amethod described in Brown et al., The Mathematics of Statistical MachineTranslation: Parameter Estimation, Computational Linguistics,19(2):263-311, 1993. In particular, it is built upon the Model 1translation model constructed in Brown. However, embodiments of thepresent invention modify the method in Brown to improve the resultingtranslation probability estimates.

At step 200 of FIG. 2, the translation probabilities are initialized.Under the prior art, the translation probabilities were initialized to auniform distribution over the target language vocabulary. The unspokenjustification for this is that EM training of the translationprobabilities will always converge to the same set of parameter valuesfrom any set of initial values, so the initial values should not matter.However, the present inventor believes that the parameters obtained atconvergence do not produce the most accurate word alignments. As such,under some embodiments of the present invention, the parameters areprevented from reaching the values that would theoretically be obtainedthrough EM training. In some embodiments, this is done by not performingthe EM algorithm to convergence. In other embodiments, this is done byreserving probability mass for unseen data.

Because the parameters are selected before convergence, theinitialization point for the parameters can affect their final value. Assuch, under some embodiments of the present invention, the translationprobabilities are initialized so that they better approximate theexpected translation probabilities rather than using uniformdistributions.

Under one specific embodiment, the translation probabilities areinitialized by converting a word association score or word alignmentscore that is not a probability into a probability estimate. The wordassociation/alignment score provides a measure of the relationshipbetween a word in one language of a bilingual training corpus and a wordin the second language of the training corpus. In the presentapplication, a word association score differs from a probabilityestimate in that sum of the word association scores for all possibleevents, including events that are not seen in a set of training data,does not have to equal one as it does for a probability estimate. Infact, many individual word association scores can be greater than one.

Under some embodiments, the word association scores for pairs of wordsthat occur infrequently are always substantially lower than those forthe highest scoring pairs of words that occur more frequently insentence pairs in the corpus. Under one particular embodiment, the wordassociation score is a log-likelihood-ratio (LLR) statistic. Thisstatistic is defined as: $\begin{matrix}{{{LLR}( {t❘s} )} = {\sum\limits_{{t?} \in {({t,{⫬ t}})}}{\sum\limits_{{s?} \in {({s,{⫬ s}})}}{{c( {{t?},{s?}} )}{\log\lbrack \frac{p( {{t?}❘{s?}} )}{p( {t?} )} \rbrack}}}}} & {{EQ}.\quad 3}\end{matrix}$

Where t represents an occurrence of a target word in a target sentence,

t represents the lack of target word t in a target sentence, srepresents the occurrence of a source word in a source sentence,

s represents the lack of source word s in a source sentence, t? is avariable that can have values of either t or

t, s? is a variable that can have values of either s or

s, c(t?,s?) is a count of the number of sentence pairs in which thecombination of the occurrence/lack of occurrence of t and s appears atleast once, p(t?|s?) is the probability of the occurrence/lack ofoccurrence of word t given the occurrence/lack of occurrence of word sin a sentence pair, and p(t?) is the probability of the occurrence/lackof occurrence of word t in a sentence.

The probability p(t?|s?) is estimated by dividing the number of sentencepairs in which there is an occurrence/lack of occurrence of t and anoccurrence/lack of occurrence of s by the number of sentence pairs inwhich there is an occurrence/lack of occurrence of S. The probabilityp(t?) is estimated by dividing the number of sentence pairs in whichthere is an occurrence/lack of occurrence of t by the number of sentencepairs in the corpus.

Using equation 3, a separate LLR score is determined for each targetword-source word pair. In cases where this results in attempting toevaluate an expression of the form 0·log(0), which is normallyundefined, the value of that expression is taken to be 0.

Word pairs can have high LLR scores if they are strongly negativelyassociated as well as if they are strongly positively associated. Sinceonly positively associated words are good candidates for being mutualtranslations, we discard any word pair whose observed probability ofoccurring together is no more than that expected by chance alone, byrequiring that p(t,s)>p(t)·p(s).

In some embodiments of the present invention, word pairs with an LLRscore lower than a certain threshold may be discarded and not consideredany further as possible translations of each other.

To improve the distribution of the word association scores, oneembodiment of the present invention raises the word association scoresto a power of n, where n is any positive real number. Thus, n may havean integer component and a fractional component. If n is greater thanone, this step increases the variance of the word association scores. Ifn is between zero and one, this step reduces the variance of the wordassociation scores. In most embodiments, a value of n that is greaterthan one is used to further differentiate scores associated with rarewords from scores associated with frequent words. This is especiallyuseful if the scores are converted into probabilities because theincrease in the variance helps to keep the probability associated withrare words relatively low.

Once the word association score has been generated, it is converted intoa probability estimate under some embodiments of the invention. To dothis, the range of scores must be transformed into a range between zeroand one. One way to do this is to separately normalize the scoresassociated with different source words. For a selected source word, eachscore that is associated with that source word is divided by the sum ofall of the scores associated with that source word.

Under a second embodiment, the scores are normalized using a singlenormalization factor for all of the source words. By doing this, it ispossible to map the relative distribution of the scores intoprobabilities such that rare source words that produce low wordassociation scores have low estimated translation probabilities. If adifferent normalization factor is used for each source word, as found inthe first embodiment, the estimated translation probabilities associatedwith rare source words can become large even if the word associationscores for the source word are small.

Under one embodiment, the single normalization factor is calculated bydetermining the sum of the scores for each source word and selecting thehighest sum as the normalization factor. This will cause theprobabilities associated with the highest sum source word to sum to one,while the probabilities associated with all other source words will sumto less than one. For rare source words, the probabilities will sum tomuch less than one, thereby placing a substantial amount of probabilitymass in reserve to account for associations between the rare source wordand target words that were not seen in the training data.

Under one embodiment, the probabilities estimated from the wordassociation scores are improved upon to take into consideration thepossibility that a word in the target language will not align with aword in the source language. This is done by assuming that there is anull or empty token in each sentence of the source language that may bealigned with any word in the corresponding sentence in the targetlanguage. Such a null token represents a lack of a word in the sourcelanguage that corresponds to a word in the target language.

Many word association metrics that are appropriate for estimatinginitial word translation probabilities for actual words are notappropriate for the null token. For instance, an LLR score for anytarget word aligning with a null source token is always 0. This occursbecause every sentence contains a null token thereby causing p(t|s) toequal p(t). Therefore, if LLR scores for the null token are transformedinto probabilities by the method described above, all initialtranslation probabilities for the null token will be 0.

Therefore, in some embodiments of the present invention, a wordassociation score is not determined for the null tokens. Instead, theinitial probability of a target word aligning with a null token is setequal to a predetermined probability associated with the target word.

Under one embodiment, the predetermined probability for the target wordis a uniform distribution across all of the words in the targetlanguage. Under a different embodiment, the predetermined probabilityfor the target word is the unigram probability of the target word asestimated from the corpus. By using the unigram probabilitydistributions, it is thought that the null token alignments will betterreflect the expectation that null tokens often align with frequentfunction words such as prepositions or determiners.

Under some embodiments, the probabilities for the null tokens arefurther modified to improve performance. In one embodiment, theprobabilities are multiplied by a constant. This increases theprobabilities of the null tokens relative to the other source words andhas been found by the present inventor to provide better alignments.

The probabilities estimated from the word association scores may be useddirectly as the final translation probabilities or may be improved uponusing an iterative algorithm such as an Expectation Maximizationalgorithm. If the probabilities are to be improved upon, the processcontinues at step 202, where the pairs of aligned sentences in thetraining corpora are used to develop counts for possible wordtranslations. In particular, for a given pair of sentences, an originalword/translated word pair is formed for each possible pairing of a wordin the sentence of the source language with a word in the sentence ofthe target language. For example, if the sentence of the target languagecontains words A, B, and C and the sentence of source language containswords U, V, and W, the word pairs UA, VA, WA, UB, VB, WB, UC, VC, and WCwould be formed. In addition, since it is possible that a word in asentence of the target language will not correspond to any word ofsource language, a null token “*” is provided as a possible word of thesource language. As a result, the additional pairs *A, *B, and *C areformed.

For each word pair in each sentence pair, a normalized count isdetermined. In particular, the normalized count is calculated as:$\begin{matrix}{{c( {{{t❘s};T},S} )} = {{J \cdot K}\frac{{tr}( {t❘s} )}{\sum\limits_{i = 0}^{l}{{tr}( {t❘s_{i}} )}}}} & {{EQ}.\quad 4}\end{matrix}$where c(t|s;T,S) is the normalized count for the target word t andsource word s in target sentence T and source sentence S, tr(t|s_(i)) isthe current estimate of the translation probability for translating theith word in the source sentence into t, K is the total number of timesword s appears in the source sentence, J is the total number of timesword t appears in the target sentence, l is the total number of actualwords in the source sentence, and s₀ is the null token. The normalizedcounts for each word pair in each sentence pair are summed to givenormalized counts c(t|s) for each word pair over all training sentencepairs.

For the first iteration, the translation probabilities are the initialtranslation probabilities set in step 200. During the first iteration ofstep 202, the translation probabilities for the null token may or maynot be modified by an adjustment value. Experiments suggest that if theinitial translation probabilities are estimated from word associationscores as described previously, it is best not to modify the null tokentranslation probabilities, but if the initial translation probabilitiesare set to a uniform distribution, it is best to multiply all the nulltoken translation probabilities by an adjustment value for the firstiteration of step 202.

At step 204, the counts for the various word pairs in the trainingsentences are used to update the translation probabilities. Under someembodiments of the present invention, the translation probabilities aresmoothed to prevent the model from becoming too confident about thetranslation probabilities for rare source words on the basis of verylittle evidence. Under one embodiment, this smoothing is achieved byadding virtual counts to the counts observed in the training data. Thisis known as add-n smoothing. In particular, the translationprobabilities are estimated as: $\begin{matrix}{{{tr}( {t❘s} )} = \frac{n + {c( {t❘s} )}}{{n{V}} + {\sum\limits_{i = 1}^{W}{c( {t_{i}❘s} )}}}} & {{EQ}.\quad 5}\end{matrix}$where tr(t|s) is the translation probability for source word sgenerating target word t, c(t|s) is the normalized count for word pair“st” summed over all sentence pairs (as described above), c(t|s) is thenormalized count for word pair “st_(i)” summed over all sentence pairs,“t_(i)” is the ith distinct word in the target language, n is an addedvirtual count, |W| is the size of the target language vocabulary Wobserved in the training data, and |V| is the hypothesized size of thetotal target vocabulary V, which may include words not seen in thetraining data. Under one embodiment, |V| is 100,000. The value of n isempirically optimized on annotated development test data. The numeratorof the right hand side of Equation 5 provides a smoothed expected countfor the source word-target word pair, while the denominator provides asmoothed expected count for the number of times the source word producesany target word.

Under some embodiments of the present invention the value of n may be sosmall as to have no significant effect on the numerator in Equation 5,only having a significant effect on the denominator, due to beingmultiplied by |V|. In this case, embodiments of the present inventionmay replace Equation 5 with Equation 6: $\begin{matrix}{{{tr}( {t❘s} )} = \frac{c( {t❘s} )}{A + {\sum\limits_{i = 1}^{W}{c( {t_{i}❘s} )}}}} & {{EQ}.\quad 6}\end{matrix}$In this equation, the number A replaces n·|V| and is empiricallyoptimized on annotated development test data. This has the advantagethat there is no need to estimate n and |V| independently.

Workers skilled in the art will recognize that there are many othermethods of smoothing the translation probabilities to prevent the modelfrom becoming too confident about the translation probabilities for raresource words on the basis of very little evidence. These includeabsolute discounting, Jelinek-Mercer smoothing, and other methods usedin statistical language modeling that do not depend on having integercounts. (See Chen, S. F., and Goodman, J. 1999. An empirical study ofsmoothing techniques for language modeling. Computer Speech and Language13 (Oct.), 359-394.)

Under the prior art, the translation probabilities associated with nullsource words have tended to be too low resulting in poor alignmentsbecause target words that should be aligned with the null source wordare instead being forced into an alignment with an actual source word.The present inventor believes that one reason for the low number ofalignments for null source words is that only a single null word ishypothesized for each source sentence. As a result, if a sentence paircontains multiple target words that should be aligned with the nullword, the null word will be under counted in comparison to actual sourcewords. For example, if an English source sentence contains two instancesof “of” and a corresponding French target sentence contains fiveinstances of “de”, the word “of” will receive two counts for every onecount for null. Thus, the probability will move toward favoring aligning“de” to “of” over the null token.

To address this, under one embodiment of the present invention, thetranslation probabilities for the null tokens are increased from thevalues determined from the counts. Under one specific embodiment, eachtranslation probability is multiplied by a value to produce a modifiedprobability.

Under some embodiments, this involves multiplying each null translationprobability by a constant that is set for the entire training corpus. Inother embodiments, the translation probabilities for the null tokens aremultiplied by a value that is selected based on the length of theparticular sentence in which the null token is found. Thus, thetranslation probabilities for null tokens in longer sentences will bymultiplied by larger values than translation probabilities for nulltokens in shorter sentences under the assumption that more null tokensare likely to be present in longer sentences than in shorter sentences.

Under many embodiments, the value that is multiplied by the translationprobability is a non-integer value and as such may have a fractionalportion such as (2.63).

At step 206, the translation probabilities are examined to determine ifmore iterations are required. If more iterations are required, theprocess returns to step 202 where a normalized count for each targetword/source word pair is determined once again for each sentence pairusing the newly estimated translation probabilities. The process repeatssteps 204, 206, and 202 until no more iterations are required, and theprocess of estimating the translation model ends at step 208.

Under one embodiment of the present invention, iterations are repeateduntil the translation probabilities have stabilized to within some smallthreshold. This is what is meant by iterating to convergence. Under thisembodiment, the final set of probabilities constitutes the trainedmodel.

Under another embodiment, a model is saved after each iteration andtested on annotated development test data. Iterations are repeated untila clear trend is established that further iterations will degradealignment accuracy on the development test data. At that point, themodel produced by any of the iterations that is most accurate on thedevelopment test data is selected as the trained model.

FIG. 3 provides a flow diagram of a method of estimating translationprobabilities under one specific embodiment of the present invention.The process of FIG. 3 begins at step 300 where a word association scoreis generated for each word pair that can be formed from the words in thesource language and the target language. For example, alog-likelihood-ratio can be determined for each word pair. Word pairswith log-likelihood-ratio scores less than a minimum threshold, e.g.,0.9, may be dismissed from further consideration. At step 302, the wordassociation scores are modified by raising them to a power of n, where nis between 1.3 and 1.7. After step 302, the modified word associationscores are converted into probability estimates at step 304. This isdone by summing the modified word association scores for each sourceword and identifying the source word with the largest sum. Each modifiedword association score is then divided by the largest sum. At step 306,probabilities for alignments between target words and null source wordsare set equal to the unigram probabilities of the target words. Theprobabilities for the null source words are then multiplied by anadjustment value at step 308. Under one embodiment, this adjustmentvalue is 2.4. After step 308, the process ends at step 310 with thetranslation probabilities estimated at steps 304 and 308 being used asthe final translation probabilities.

FIG. 4 provides a flow diagram of a second specific method forestimating translation probabilities. In step 400 of FIG. 4, a wordassociation score is determined and at step 402 the word associationscore is modified by raising it to a power of n. Under one embodiment,the word association score is an LLR score and it is raised to the powerof 1.5. At step 404, the modified word association scores are convertedinto probabilities by dividing each modified word association score bythe largest sum of the modified word association scores associated withany one source word. Probabilities for translations involving a nulltoken are then estimated at step 406 by using the unigram probability ofthe target word involved in the translation as the translationprobability.

At step 408, weighted counts are determined for each translation wordpair using equation 4 above.

At step 410, the weighted counts are used to estimate translationprobabilities for each word pair using the add-n smoothing of equation 5above. Under one embodiment, a value of 0.0005 is used for n. In otherembodiments, n is set to zero. When estimating the translationprobabilities for null tokens, the probabilities are first estimatedusing Equation 5. Those translation probabilities are then adjusted.Under one embodiment, the translation probabilities are adjusted bymultiplying the translation probabilities estimated using Equation 5 byan adjustment value of 10.0.

After the probabilities have been estimated at step 410, the processdetermines if more iterations should be performed at step 412. Under oneembodiment, only a single iteration is performed before the process endsat step 414. In other embodiments, multiple iterations are performed byreturning to step 408 to update the weighted counts. Steps 408, 410, and412 are repeated until the desired number of iterations have beenperformed.

FIG. 5 provides a flow diagram of a third specific method under thepresent invention for estimating translation probabilities. In step 500,the translation probabilities are initially set to the same uniformdistribution value. Under one embodiment, the probabilities for the nulltoken are then multiplied by an adjustment value of 10.0. At step 502,weighted counts are determined using equation 4 above. At step 504, theweighted counts are used to estimate new values for the translationprobabilities while using the add-n smoothing of Equation 5. Under oneembodiment, n is equal to 0.01. The translation probabilities for thenull tokens are estimated by first using Equation 5 to form atranslation probability and then adjusting that translation probability.Under one embodiment, the translation probability is adjusted bymultiplying the translation probability by an adjustment value of 10.0.

At step 506, the process determines if more iterations are to beperformed. Under one embodiment, 14-20 iterations are performed. If moreiterations are to be performed, the process returns to step 502 todetermine new values for the weighted counts and steps 504 and 506 arerepeated. Once enough iterations have been performed at step 506, theprocess ends at step 508.

Although the present invention has been described with reference toparticular embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

1. A method of estimating translation probabilities, the methodcomprising: determining word association scores for a plurality of wordpairs, each word pair comprising a source word in a source language anda target word in a target language; selecting at least one normalizationvalue; and converting the word association scores into translationprobabilities using the normalization value such that the sum of all ofthe translation probabilities for at least one source word is less thanone.
 2. The method of claim 1 wherein selecting a normalization valuecomprises: for each source word, summing the word association scoresassociated with the source word to determine a total word associationscore for the source word; and selecting the largest total wordassociation score as the normalization value.
 3. The method of claim 1further comprising estimating translation probabilities for null tokens.4. The method of claim 1 wherein estimating translation probabilitiesfor null tokens comprises estimating unigram probabilities for targetwords and for each target word, using the unigram probability for thetarget word to estimate the translation probability for translating froma null token to the target word.
 5. The method of claim 4 whereinestimating a translation probability for a null token further comprisesmodifying a unigram probability for a target word to estimate thetranslation probability.
 6. The method of claim 5 wherein modifying aunigram probability to estimate a translation probability comprisesmultiplying the unigram probability by an adjustment value.
 7. Themethod of claim 1 further comprising raising each word association scoreto a power before converting the word association scores intotranslation probabilities.
 8. A computer-readable medium havingcomputer-executable instructions for performing steps comprising:determining a word association score for a source word and a targetword; converting the word association score into a translationprobability using a normalization value that reserves some probabilitymass.
 9. The computer-readable medium of claim 8 further comprisingidentifying the normalization value through steps comprising: for eachsource word in a plurality of source words, summing the word associationscores for the source word to form a total word association score foreach source word; and selecting the largest total word association scoreas the normalization value.
 10. The computer-readable medium of claim 8further comprising estimating a translation probability for a nulltoken.
 11. The computer-readable medium of claim 10 wherein estimating atranslation probability for a null token comprises estimating a unigramprobability for a target word.
 12. The computer-readable medium of claim11 wherein estimating a translation probability for a null token furthercomprises modifying the unigram probability for the target word toestimate the translation probability.
 13. The computer-readable mediumof claim 12 wherein modifying the unigram probability comprisesmultiplying the unigram probability by an adjustment value.
 14. Thecomputer-readable medium of claim 8 wherein converting the wordassociation score into a probability comprises raising the wordassociation score to a power.
 15. A computer-readable medium havingcomputer-executable instructions for performing steps comprising:determining a set of word association scores for word pairs, whereineach word pair comprises a word in a first language and a word in asecond language; and raising each word association score to a power. 16.The computer-readable medium of claim 15 wherein raising each wordassociation score to a power comprises raising each word associationscore to a power that is greater than one.
 17. The computer-readablemedium of claim 15 wherein raising each word association score to apower comprises raising each word association score to a power thatcomprises a fractional component.
 18. The computer-readable medium ofclaim 15 wherein determining a set of word association scores comprisesdetermining a set of log-likelihood-ratio scores.
 19. Thecomputer-readable medium of claim 15 wherein raising the set of wordassociation scores to a power generates adjusted word association scoreswherein the adjusted word association scores have a greater variancethan the word association scores.